Special low impedance output circuit for high chain bridge amplifier



Jan. 17, 1956 R. w. BORDEWIECK 2,731,519

SPECIAL LOW IMPEDANCE OUTPUT CIRCUIT FOR HIGH CHAIN BRIDGE AMPLIFIER Filed June 22, 1950 INVEN TOR. fflf mz cclf United States Patent SPECIAL LOW IMPEDANCE OUTPUT CIRCUIT FOR HIGH CHAIN BRIDGE AMPLIFIER Robert W. Bordewieck, Southboro, Mass., assignor to Moore Electronic Laboratories, Incorporated, Worcester, Mass., a corporation of Massachusetts Application June 22, 1950, Serial No. 169,723

2 Claims. (Cl. 179-171) This invention relates to a special low impedance output circuit for a high gain bridge amplifier and the principal object of the invention resides in the provision of a circuit which allows the bridge amplifier to be operated for maximum D. C. gain and allows all of the components to be easily determined for maximum voltage gain. The novel special output circuit takes the output from the high gain amplifier and reduces the impedance to a relatively very low value to drive a relay, a meter, a recorder, or any similar electromechanical device.

Other objects and advantages of the invention will appear hereinafter.

Reference is to be had to the accompanying drawings in which Fig. l is a circuit diagram showing the invention; and

Figs. 1A and 1B are fragmentary showings of different devices operated thereby.

Fig. 1 shows one form of the invention wherein numeral represents a source of electrical energy having terminals 12 and 14 which supply heater voltage to bridge amplifier tubes 30 and 40 through connections X X to heaters X, X. Numerals 16 and 18 are terminals used to supply heater power to the filaments of the output tubes 70 and 80 through connections Y Y to heaters Y, Y. It is preferred that tubes 30 and 40 be of a twin triode or a twin pentode design, and that tubes 70 and 80 be of a twin triode or a twin pentode design.

Source of electrical energy 10 also supplies high voltage at terminal 22 connected by lines 23, 24, 25 to the plates 72 and 82 of the output stage; and by lines 23 and 26 to ballast resistor 27 and thence by line 90 to voltage regulator tubes 91 and 93 and thence to the B minus return.

Plate load resistors 31 and 41 of tubes and respectively are connected to the source of regulated high voltage at the point 95 and to the plates 32 and 42 of tubes 30 and 40 respectively. Grids 34 and 44 are connected through grid resistors 35 and respectively to B minus terminal 20 through lines 50, 51, 52, 53 and 54. Cathodes 36 and 46 are connected to opposite ends of potentiometer 28 whose slider 28A is connected through resistor 29 to the same B minus return line 51.

The output of this bridge amplifier is taken from the plates as is the usual practice with bridge amplifiers. However, because of the high impedance of the plate circuit and the desire for a low impedance output, line 38 is connected from plate 32 to grid 74, and plate 42 is connected by line 48 to grid 84 of the output stage. In order to give this output stage a designation and partially identify its function, it will be referred to as an unbalanced cathode follower bridge output stage, and one cathode 86 does not require any cathode resistor of its own. Cathode 76 of the unbalanced cathode follower bridge output stage is connected through resistor 78 and through lines 53 and 54' to the B minus return at 20. The magnitude of resistor 78 is determined by the output impedance which is desired.

A meter 96 is connected as by line 98 to the cathode Z,731,5 i9 Patented Jan. 17, 1956 86 and by line 100 to the cathode 76. An optional output circuit is shown in Fig. 1A, wherein, a relay coil 102 is connected in place of meter 96. In Fig. 1B a recorder or any other electro-magnetically driven means 104 is shown with the same connections to the output of the amplifier.

The operation of this circuit is as follows: The heaters X, X of tubes 30 and 40 are operated at a reduced voltage to eliminate gas current effects, reduce contact potential variations and decrease the efiects of grid current and grid emission, etc. Bridge amplifier tubes 30 and 40 are operated with extremely high plate load resistors 31 and 41, being on the order of 2 megohms, thus providing a very high gain amplifier. If pentodes were used for tubes 30 and 40, high plate load resistors would again be used and both the screen and the plate supplies would be regulated. Voltage regulator tubes 91 and 93 prevent variations in the high voltage supply from affecting the operation of the amplifier. The balanced nature of a bridge amplifier maintains the output voltage between plates 32 and 42 constant despite variations in heater voltage, temperature, humidity, shock, vibration, etc., in so far as this is possible.

Because of slight differences in tubes 30 and 40 even though they are mounted in the same envelope, it is desirable to employ a cathode method of compensating the amplifier to bring its plates to equal potential. If the plate load resistance were used to accomplish this result, the circuit would have been greatly unbalanced under signal conditions and output would not have been as nearly linear.

A common cathode resistor 29 is used to provide degeneration in amplifier tube 30 and the resulting de-generation voltage developed across resistor 29 is impressed as a signal on grid 44 of tube 40. Thus when a signal voltage of positive polarity is impressed on input 106 to grid 34, it results in a negative signal being applied to grid 44 through the action of de-generation of resistor 29. Grid resistors 35 and 45 are preferably equal to help maintain balance, although it was found that leaving resistor 45 at 10 megohms, resistor 35 could be changed from 20 megohms to zero without upsetting the balance of the bridge amplifier, when a 6N7 tube is used for 30 and 40 and its heater voltage is ballast regulated to three volts.

The circuit shown provides for input signals of a positive polarity to be applied to input 106 or negative signals to be applied to input 108. If a positive signal is applied to grid 34 from input 106, the current in tube 30 increases, the plate voltage 32 drops, the cathode current through resistor 29 increases, and a resultant rise in voltage across resistor 29 occurs. This causes grid 44 to become relatively more negative with respect to its cathode 46, and tube 40 conducts less plate current, thus raising the voltage on its plate 42. Thus the voltage impressed on grid 74 of tube 70 through line 38 drops, and the voltage impressed on grid 84 of tube rises. This results in a decrease in the plate current of tube 70 and an increase in the plate current of tube 80 and therefore in the meter 96 or other device connected between cathodes 86 and 76 the current also increases.

If it is assumed that the bridge amplifier tubes are balanced, it will be seen than an appreciable current passes through both cathode follower tubes 70 and 80 to set up the initial bias for the cathode follower output stage across resistor 78. This means that an appreciable initial current will flow through meter 96 with no signal. If a positive signal is applied to input 106, the current through cathode follower tube 70 decreases and the current through cathode tube 80 increases causing the resulting output current through meter 96 to increase in accordance with the increase of signal applied to input 1; 106. Under usual conditions bridge amplifier tubes 30 and 4-0 are intentionally unbalanced by means of the cathode biasing network so that the initial current through meter 96 is zero or nearly zero, and the circuit then lends itself as a linear amplifier whose output current is approximately directly proportional tothe input voltage.

On the other hand, if it is desired to use a relay in the output circuit ofthe amplifier, the initial current flowing through the relay coil 102 is made just under the drop out current of the relay. Therefore, an increase in signal to input 106 willcause the relay 102 to pull on and will require that the signal be reduced to almost zero in order that the relay may again drop out; If a recorder or similar instrument is used, the initial current may or may not be eliminated. In many cases the initial current will enable the use of a more rugged output device, inthat more power will be available to the output if the initial current-is an appreciable value. That is, power output depends upon the square of the current; a change of 1 milliampere-into 1000 ohm load coil will produce one milliwatt of power variation if the initial current is zero. However, if the initial current is 10 milliamperes and the output increases to 11 milliamperes, the output power will have changed from .100 watt to .121 watt. Thus it will be seen that for the same change of 1 milliampere in output current-in one case, there is obtained a power of l milliwatt to perform the work or" moving a pin or other indicator, whereas in the second case, 21 rnilliwatts are available from the same 1 milliampere change in current.

The input signal may be impressed between inputs 106 and 108; or it may be impressed from 106 to 106A; or it may be impressed from 108 to 108A. Link 110 may be inserted to ground the B minus return at contact 112 to ground point 114 or the amplifier may be left ungrounded. The numerals 107 and 109 respectively indicate the leads to the inputs 106A and 108A connecting the same to the B minus terminal 20.

Having thus described my invention and the advantages thereof, I do not wish to be limited to the details herein disclosed otherwise than as set forth in the claims, but what I claim is:

1. The combination with a bridge amplfier circuit of a special low impedance output, said circuit comprising a source of voltage, two cathode follower tubes and two bridge amplifier tubes, the cathode follower tubes being connected by their grids to the plates of the bridge amplitier tubes and the first cathode follower tubeonly having the cathode thereof connected through a cathode load resistor to' the low voltage return of said source of voltage, the cathode of the second cathode follower tube being connected through an electro-mechanically driven device to the cathode of the first cathode follower tube.

2. The combination with a voltage supply, a pair of bridge amplifier tubes, a pair of cathode follower tubes, the grids of the latter being connected separately to the plates of the former, means to adjust the voltage balance at the bridge amplifier plates to-a value other than zero, a cathode load resistance for the cathode of one cathode follower tube,-the cathode of the other cathode follower tube operating in the absence of a cathode load resistance, of an electro-mechanical device connected to and between the cathodes of the cathode follower tubes, the said cathode load resistance substantially matching the impedance of the device, and input connections to the tubes of the bridge amplifier tubes.

References Cited in the file of this patent Publication, Vacuum Tube Amplifier, Valley, Jr., and Wallman, Radiation Laboratory Series, 1948 edition, Mc- Graw-Hill Book Co., publisher, p. 481, Figure 11.64. 

